Permeability of micro-scale structure in coal: Insights from μ-CT image and pore network modelling

Abstract

For developing coalbed methane resources, it is necessary to quantify the heterogeneity embodied in pore size and spatial distribution, and on the basis of which the correlation between the gas transportability and pore structure could then be clarified in coal. In this study the μ-CT imaging technique was used to examine the micro-scale 3D pore structure in two Qinshui basin coals, and the permeability was estimated based on the reconstructed real pore structure. With CT image processing and 3D pore structure, the pore size, connectivity and the pore spatial distribution were displayed, and the connected pores and isolated pores were distinguished, and the proportion of connected pores of the two studied coals was estimated to be 54.22%–68.52% and 79.48%–91.86%, respectively. The surface porosity of 2D structure slice interprets the local pore connectivity, the minimum surface porosity in certain direction of 3D structure was close to 0 for the studied coals, which means that the pore structure is not connected and it has poor permeability in the direction of coal seam. The gas flow path and flow fluxe in 3D structure was visualized and quantified by gas transport simulation based on real coal pore structure, and the permeability and its anisotropy was evaluated. It indicates that the gas pressure appears to be distributed in a “cluster” shape, and the heterogeneity could be major reason for the low gas transport capacity in coal. The permeability of the two studied coals in the horizontal direction is significantly higher than that in the vertical direction, because the transport channels with low surface porosity in the vertical direction have the tendency to hinder the gas transport.